CS2844/3845 SERIES
CS2844/CS3844
CS2845/CS3845
Current Mode PWM Control Circuit
with 50% Max Duty Cycle
Description
The CS3844/45 provides all the neces-
sary features to implement off-line
fixed frequency current-mode control
with a minimum number of external
components.
The CS3844 family incorporates a new
precision temperature-controlled oscil-
lator to minimize variations in fre-
quency. An internal toggle flip-flop,
which blanks the output every other
clock cycle, limits the duty-cycle range
to less than 50%. An undervoltage
lockout ensures that V
REF
is stabilized
before the output stage is enabled. In
the CS2844/CS3844 turn on occurs at
16V and turn off at 10V. In the
CS2845/CS3845 turn on is at 8.4V and
turn off at 7.6V.
Other features include low start-up
current, pulse-by-pulse current limit-
ing, and a high-current totem pole out-
put for driving capacitive loads, such
as gate of a power MOSFET. The out-
put is low in the off state, consistent
with N-channel devices.
Features
s
Optimized for Offline
Control
s
Temp. Compensated
Oscillator
s
50% Max Duty-cycle Clamp
s
V
REF
Stabilized before
Output Stage is Enabled
s
Low Start-up Current
s
Pulse-by-pulse Current
Limiting
s
Improved Undervoltage
Lockout
s
Double Pulse Suppression
s
1% Trimmed Bandgap Ref.
s
High Current Totem Pole
Output
Absolute Maximum Ratings
Supply Voltage (I
CC
<30mA).........................................................................Self Limiting
Supply Voltage (Low Impedance Source) .................................................................30V
Output Current..............................................................................................................±1A
Output Energy (Capacitive Load) ................................................................................5µJ
Analog Inputs (V
FB
, V
SENSE
) .........................................................................-0.3V to 5.5V
Error Amp Output Sink Current .............................................................................10mA
Package Options
8 Lead PDIP & SO Narrow
COMP
V
FB
Sense
OSC
1
2
3
4
8
7
6
5
Lead Temperature Soldering
Wave Solder (through hole styles only) ...................10 sec. max, 260°C peak
Reflow (SMD styles only) ....................60 sec. max above 183°C, 230°C peak
Block Diagram
V
REF
V
CC
V
OUT
Gnd
14 Lead SO Narrow
V
CC
V
CC
Undervoltage Lock-out
COMP
1
14
13
12
11
10
9
8
V
REF
NC
V
CC
V
CC
Pwr
V
OUT
Gnd
Pwr Gnd
V
CC
Pwr
NC
2
V
FB
3
34V
Gnd
16V/10V
(8.4V/7.6V)
V
FB
Error
Amplifier
-
+
OSC
2.50V
Set/
5.0 Volt
Reset Reference
Internal
Bias
R
V
REF
Undervoltage
Lockout
Toggle
Flip-Flop
V
REF
NC
4
Sense
5
NC
6
OSC
7
COMP
16 Lead SO Wide
NC
1
NC
2
16
15
14
13
12
11
10
9
NC
V
REF
V
CC
V
CCPwr
V
O
Gnd
Pwr Gnd
NC
R
NOR
V
OUT
COMP
3
V
FB
4
Oscillator
2 R
R
Sense
1V
S
Pwr Gnd
R
Current PWM
Sensing Latch
Comparator
SENSE
5
OSC
6
NC
7
NC
8
( ) indicates CS-2845/3845
Cherry Semiconductor Corporation
2000 South County Trail, East Greenwich, RI 02818
Tel: (401)885-3600 Fax: (401)885-5786
Email: info@cherry-semi.com
Web Site: www.cherry-semi.com
Rev. 3/17/99
1
A
®
Company
CS2844/3845 SERIES
Electrical Characteristics: -25
≤
T
A
≤
85˚C for CS2844/2845, 0
≤T
A
≤
70˚C for CS3844/3845. V
CC
= 15V (Note 1); R
T
= 10kΩ,
C
T
= 3.3nF for sawtooth mode., unless otherwise stated.
CS2844/CS2845
MIN TYP MAX
CS3844/CS3845
MIN TYP MAX UNITS
PARAMETER
TEST CONDITIONS
s
Reference Section
Output Voltage
Line Regulation
Load Regulation
Temperature Stability
Total Output Variation
Output Noise Voltage
Long Term Stability
Output Short Circuit
s
Oscillator Section
Initial Accuracy
Voltage Stability
Temperature Stability
Amplitude
s
Error Amp Section
Input Voltage
Input Bias Current
A
VOL
Unity Gain Bandwidth
PSRR
Output Sink Current
Output Source Current
V
OUT
HIGH
V
OUT
LOW
s
Current Sense Section
Gain
Maximum Input Signal
PSRR
Input Bias Current
Delay to Output
s
Output Section
Output Low Level
Output High Level
Rise Time
Fall Time
I
SINK
=20mA
I
SINK
=200mA
I
SOURCE
=20mA
I
SOURCE
=200mA
T
J
=25˚C, C
L
=1nF (Note 2)
T
J
=25˚C, C
L
=1nF (Note 2)
13.0
12.0
0.1
1.5
13.5
13.5
50
50
150
150
0.4
2.2
13.0
12.0
0.1
1.5
13.5
13.5
50
50
150
150
0.4
2.2
V
V
V
V
ns
ns
(Notes 3 & 4)
V
COMP
=5V (Note 3)
12≤V
CC
≤25V
(Note 3)
V
Sense
=0V
T
J
=25˚C (Note 2)
2.85
0.9
3.00
1.0
70
-2
150
-10
300
3.15
1.1
2.85
0.9
3.00
1.0
70
-2
150
-10
300
3.15 V/V
1.1
V
dB
µA
ns
V
COMP
=2.5V
V
FB
=0V
2≤V
OUT
≤4V
(Note 2)
12≤V
CC
≤25V
V
FB
=2.7V, V
COMP
=1.1V
V
FB
=2.3V, V
COMP
=5V
V
FB
=2.3V, R
L
=15kΩ to Gnd
V
FB
=2.7V, R
L
=15kΩ to V
REF
65
0.7
60
2
-0.5
5
2.45
2.50
-0.3
90
1.0
70
6
-0.8
6
0.7
1.1
2.55
-1.0
65
0.7
60
2
-0.5
5
2.42
2.50
-0.3
90
1.0
70
6
-0.8
6
0.7
1.1
2.58 V
-2.0
µA
dB
MHz
dB
mA
mA
V
V
Sawtooth Mode, T
J
=25˚C
12≤V
CC
≤25V
Sawtooth Mode T
MIN
≤T
A
≤T
MAX
(Note 2)
V
OSC
(peak to peak)
47
52
0.2
5
1.7
57
1.0
47
52
0.2
5
1.7
57
1.0
kHz
%
%
V
T
J
=25˚C, I
REF
=1mA
12≤V
CC
≤25V
1≤I
REF
≤20mA
(Note 2)
Line, Load, Temp. (Note 2)
10Hz≤ f≤10kHz, T
J
=25˚C (Note 2)
T
A
=125˚C, 1000 Hrs. (Note 2)
T
A
=25˚C
-30
4.90
50
5
-100
25
-180
-30
4.95
5.00
6
6
0.2
5.05
20
25
0.4
5.10
4.82
50
5
-100
25
4.90
5.00
6
6
0.2
5.10 V
20
25
0.4
mV
mV
mV/˚C
µV
mV
-180 mA
5.18 V
2
CS2844/3845 SERIES
Electrical Characteristics: -25≤T
A
≤85˚C
for CS2844/2845, 0≤T
A
≤70˚C
for CS3844/3845. V
CC
=15V (Note 1); R
T
=10kΩ,
C
T
=3.3nF for sawtooth mode., unless otherwise stated.
PARAMETER
TEST CONDITIONS
CS2844/CS2845
MIN TYP MAX
CS3844/CS3845
MIN TYP MAX UNITS
s
Total Standby Current
Start-Up Current
Operating Supply Current
V
CC
Zener Voltage
s
PWM Section
Maximum Duty Cycle
Minimum Duty Cycle
CS2844
TYP MAX
0.5
V
FB
=V
Sense
=0V R
T
=10kΩ, C
T
=3.3nF
I
CC
=25mA
11
34
1.0
17
0.5
11
34
1.0
17
mA
mA
V
46
48
50
0
46
48
50
0
%
%
PARAMETER
TEST CONDITIONS
MIN
CS3844
MIN TYP MAX
CS2845/CS3845
MIN TYP MAX UNITS
s
Under-Voltage Lockout Section
Start Threshold
Min. Operating
Voltage
Notes:
15
After Turn On
9
16
10
17
11
14.5
8.5
16
10
17.5
11.5
7.8
7.0
8.4
7.6
9.0
8.2
V
V
1. Adjust V
cc
above the start threshold before setting at 15V.
2.These parameters, although guaranteed, are not 100% tested in production.
3. Parameter measured at trip point of latch with V
FB
=0.
4. Gain defined as:
A=
∆V
COMP
∆V
Sense
; 0
≤
V
Sense
≤
0.8V.
Package Pin Description
PACKAGE PIN #
PIN SYMBOL
FUNCTION
8L
PDIP/SO
1
2
3
4
5
14L
SO Narrow
1
3
5
7
9
8
16L
SO Wide
3
4
5
6
11
10
12
13
14
15
1,2,7,8,9,16
COMP
V
FB
Sense
OSC
Gnd
Pwr Gnd
V
OUT
V
CC
Pwr
V
CC
V
REF
NC
Error amp output, used to compensate error amplifier.
Error amp inverting input.
Noninverting input to Current Sense Comparator.
Oscillator timing network with Capacitor to Ground, resis-
tor to V
REF
.
Ground.
Output driver Ground.
Output drive pin.
Output driver positive supply.
Positive power supply.
Output of 5V internal reference.
No Connection.
6
10
11
7
8
12
14
2,4,6,13
3
CS2844/3845 SERIES
Test Circuit Open Loop Laboratory Test Fixture
V
REF
R
T
2N2222
100kΩ
4.7kΩ
1kΩ
Error Amp
Adjust
4.7kΩ
5kΩ
A
COMP
V
REF
0.1µF
V
CC
V
FB
V
CC
0.1µF
1kΩ
1W
Sense
Adjust
Sense
V
OUT
V
OUT
OSC
Gnd
Gnd
C
T
Circuit Description
Undervoltage Lockout
During Undervoltage Lockout (Figure 1), the output driv-
er is biased to sink minor amounts of current. The output
should be shunted to ground with a resistor to prevent
activating the power switch with extraneous leakage cur-
rents.
PWM Waveform
To generate the PWM waveform, the control voltage from
the error amplifier is compared to a current sense signal
which represents the peak output inductor current (Figure
2). An increase in V
CC
causes the inductor current slope to
increase, thus reducing the duty cycle. This is an inherent
feed-forward characteristic of current mode control, since
the control voltage does not have to change during
changes of input supply voltage.
When the power supply sees a sudden large output cur-
rent increase, the control voltage will increase allowing
the duty cycle to momentarily increase. Since the duty
cycle tends to exceed the maximum allowed to prevent
transformer saturation in some power supplies, the inter-
nal oscillator waveform provides the maximum duty cycle
clamp as programmed by the selection of OSC compo-
nents.
V
CC
ON/OFF Command
to reset of IC
CSX844
V
ON
V
OFF
16V
10V
CSX845
8.4V
7.6V
I
CC
<15mA
<1mA
V
ON
V
OFF
V
CC
Figure 1: Startup voltage for CSX844 and CSX845.
4
CS2844/3845 SERIES
Circuit Description: continued
Setting the Oscillator
V
OSC
OSC
RESET
Toggle
F/F Output
EA Output
Switch
Current
V
CC
The times T
c
and T
d
can be determined as follows:
t
c
= R
T
C
T
ln
t
d
= R
T
C
T
ln
(
(
V
REF
- V
lower
V
REF
- V
upper
)
)
V
REF
- I
d
R
T
- V
lower
V
REF
- I
d
R
T
- V
upper
I
O
Substituting in typical values for the parameters in the
above formulas:
V
REF
= 5.0V, V
upper
= 2.7V, V
lower
= 1.0V, I
d
= 8.3mA,
then
t
c
≈
0.5534R
T
C
T
t
d
= R
T
C
T
ln
V
O
Figure 2: Timing Diagram
(
2.3 - 0.0083 R
T
4.0 - 0.0083 R
T
)
For better accuracy R
T
should be
≥10kΩ.
V
upper
V
lower
t
on
t
C
t
on =
t
C
t
off =
t
C+
2t
d
Figure 3: Duty Cycle parameters.
Grounding
t
off
t
d
High peak currents associated with capacitive loads neces-
sitate careful grounding techniques. Timing and bypass
capacitors should be connected close to Gnd in a single
point ground.
The transistor and 5kΩ potentiometer are used to sample
the oscillator waveform and apply an adjustable ramp to
Sense.
5
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